Parser: jp-array-loop and jp-call-args-loop detect punct "..."
and emit (js-spread inner).
Transpile: when any element is spread, build array/args via
js-array-spread-build with (list "js-value" v) and (list
"js-spread" xs) tags.
Runtime: js-array-spread-build walks items, appending values or
splicing spreads via js-iterable-to-list (handles list/string/dict).
Works in arrays, call args, variadic fns (Math.max(...arr)),
and string spread ([...'abc']).
414/416 unit (+5), 148/148 slice unchanged.
giles
4800246b23